Try a HDMI Splitter between the cable box and the Slingbox. Do some homework and search Amazon for: HDMI Splitter" and read the reviews. Should cost in the $25 Range. Also take a look at www.tweaking4all Dot com/home-theatre/remove-hdcp-hdmi-signal/
Splitters come in two forms, one that the output is component, and another is HDMI only. I user the latter. Both should work. Be advised that there is not a standard among splitters, and this is why reading the customer review is important.
Mine works fine.
Figuring out you need an HDMI-to-component converter isn't too difficult. As stated in another post, the one recommended by Slingbox works. I can say that this one works also as I just connected my SB 350 to my new Xi3 companion box. I had to do several rests on the Slingbox, but everything works--premium channels, virtual remote (we have an X5). You may find cheaper versions that work equally well.